Comment by kenneth

It's endlessly annoying to me that the em-dash has become the canary in the coalmine of AI writing because I've always used them extremely liberally in my writing.

Honestly, this might sound elitist, but I suspect it's because it's an "advanced" punctuation that is not known by most people, so is not commonly used. But the training corpus of these models puts more weight of academic writing or published books writing where the em-dash is much more commonly used.